Basket Case RAP Transatlantic

[mickjjo]

I have just taken some voltage readings with the set working normaly:-

The AC input to the 80 rectifier is 315-0-315 volts.
The DC output at the rectifier heater/cathode and one side of the speaker field coil is 340 volts.
The DC at the other side of the field coil is 290 volts.
The HT at the anode of the 42 output valve is 275 volts. Hope this helps

(The serial No. seems to be RT 12406.)

Regards, Mick.

[PJL]

Patrick, your speaker has been replaced with a permanent magnet one. The transformer is possibly heater? The yellow bit is indeed a rectifier. It looks like the work is old possible a WWII botch?

If you go the transformer route then something from a more modern set would be best as you don't want the extra HT volts dropped by the field coil.

[Patrick Dixon]

Hi PJL,

Yes, the field coil has been replaced by the choke (not transformer) mounted where the mains transformer was. I was considering relocating it under the chassis like Radio Dave did with his 646 IIRC. I suppose I could just use a 6.3v transformer to drive the heaters for now, and leave the rectifier stack where it is. Any idea how much voltage drop to expect in such a rectifier?

There's a 300-0-300, 5V, 6.3Vct transformer here http://neutronics.ltd.uk/transformers.htm which might be suitable. I've emailed them for a better spec.

[PJL]

Patrick, firstly you need to decide which way to go...
1. Mains transformer and valve rectifier
2. Mains transformer and silicon rectifier
3. 6.3V transformer and 'live' chassis

Option 2 might be the best compromise as virtually any modern radio transformer should do the job. Stick a wanted posting in for a 250-0-250 6.3V transformer and I am sure one can be had for a much more reasonable price.

Keep the choke, scrap the rest of the PSU, two silicon rectifiers 1N4007 from transformer to choke and a couple of 16uF/450V caps either side of the choke to earth and it's done although you might need a resistor to keep the HT down a bit, a 42 anode shouldn't exceed 250V...Peter

[PJL]

I should have checked your posts properly, I had assumed it was a heater transformer as you said the lineup was a 42.

Anyway, the unmarked valve is very unlikely to be a 42 if it was in series. It would most likely be a 25a6g if it was octal but not too sure if it's UX based. All options above will need a new 42.

A fourth low cost option is to continue to use a series heater chain with a capacitor dropper or even use the barretter but it could do with a proper holder! I wouldn't recommend it as the chassis will be 'live' and the back doesn't exactly cover it!

[the_snark]

Hi,
I thought as you'd all revealed your undersides, that I would show you mine. As you can see it has been heavily modified. I think it has had a transformer transplant and some caps have been changed, all a long time ago by the looks of it. It came without valves and I have found that the easiest way to obtain these old American valves is on American ebay. The valves are cheap but the postage is dear.
Patrick, thank for your offer to send me details of the back. I think if you can take a photo for me reasonably square on, I should be able to take the outline from that.

[Patrick Dixon]

Here's a photo of the back. It's 1/4" ply. The 'bite' at the top is heat damage from the barretter.

I can easily mail you a traced outline if you prefer.

[Patrick Dixon]

Originally Posted by PJL

... although you might need a resistor to keep the HT down a bit, a 42 anode shouldn't exceed 250V...Peter

The GE datasheet at the valve museum lists 42 plate voltage between 250V and 285V for typical operation, and 415V absolute max - which fits in with Mick's set's HT at 275V.

If the rectifier valve drops 60V from a 315V secondary, 250V into a silicon rectifier with a negligible voltage drop should be about right I reckon.

[Patrick Dixon]

Originally Posted by PJL

Anyway, the unmarked valve is very unlikely to be a 42 if it was in series. It would most likely be a 25a6g if it was octal but not too sure if it's UX based. All options above will need a new 42.

On closer examination. the valve is marked as an 18 - which seems to be the same as a 42 but with a 14V 0.3A heater.